This invention relates to absorbent, retentive cellulose pulp which is capable of retaining good absorbency even after having been highly compressed. This pulp is provided for use in absorbent products such as sanitary napkins, catamenial tampons, diapers, dressings or the like which are used for absorbing body fluids.
For many years, cellulose pulp has been utilized for absorbing body fluids. Wood pulp has been found most suitable for such products primarily because it is an inexpensive, readily available absorbent material. Such wood pulp is generally derived from soft wood trees such as southern pine and the like and is commercially treated in chemical pulping processes such as the kraft or sulfite processes during which the trunks and branches of trees are reduced to wood pulp fibers and non-fibrous substances such as gums, resins and lignin are chemically removed. The resulting wood pulp is sometimes bleached and then formed into board for subsequent disassociation into pulp fluff to be used in the aforementioned products.
Although pulp fluff derived from the conventional process steps has, in the past, been successfully employed in body fluid absorption products, the art has increasingly sought to improve the absorption capacity and fluid retention properties of wood pulp. Many suggestions have already been advanced, generally directed towards chemical modifications of the cellulose polymer of which the wood pulp fibers are composed. While these efforts have met with some success, the resulting products are substantially more expensive than native wood pulp and suffer from some peculiar drawbacks such as brittleness or slow wicking rates.
It has long been known that the absorbency of cellulosic fibers may be improved by wet cross-linking the fibers. Thus, U.S. Pat. No. 3,241,553 discloses such cross-linking in order to provide absorbent fibrous products which have improved absorbency as well as the ability to retain greater amounts of absorbed fluids when subjected to pressures which tend to squeeze out the fluids absorbed. There is, however, no disclosure in said U.S. Pat. No. 3,241,553 concerning the cross-linking of microfibrillated fibers.
The need for a relatively inexpensive, simple process for treating native cellulose fibers to increase their absorption capacity and fluid retention properties has been met to a limited degree by the process disclosed by Chatterjee, et al. in U.S. Pat. No. 4,474,949. Chatterjee, et al. disclosed a process of mechanically beating a dispersion of cellulose fibers to a degree such that at least the outermost of the secondary walls of the cellulose fibers were essentially completely disintegrated to microfibrillar form followed by the freeze drying of the beaten dispersion. The resultant material possesses excellent absorption properties at low densities, but poor absorption properties at higher densities. In addition, the mechanical strength of this material is too low since it collapses in contact with water under a confining pressure.
Furthermore, the freeze drying step of Chatterjee, et al., requires a lengthy production time of about 90 hours. Accordingly, there is a need for a much faster relatively inexpensive, simple process for treating native cellulose fibers to increase their absorption capacity and fluid retention properties, not only at low densities but also at higher densities.
It is an object of the present invention to provide a thin densified and flexible material to be utilized as an insert or entirely as an ultra-thin sanitary product. This material possesses high absorption and retention properties coupled with a fast wicking rate to absorb body fluid. The material should also demonstrate a Z direction swelling in the wet state and have a high structural integrity so that it can be easily handled. The final absorption properties of the product can be varied to some extent by varying the size and amount of pore generating particles used in the process of the present invention. The expression "Z direction" as used herein is intended to signify the direction of initial compression of the compressed product.